Right now, every moment of every day, humans are slowly reshaping the Earth’s climate. Hotter summers and wetter storms. Higher oceans and more violent wildfires. The threats to our homes, our society and the environment around us are multiplying all the time.
Furthermore, we may be causing even more momentous changes in the climate.
For two decades, scientists have warned that warming due to carbon emissions could push some large systems in the natural world to collapse. These are systems so vast that they manage to maintain a certain equilibrium despite rising temperatures, but only up to a point.
Scientists say that once we warm the planet above certain levels, this balance could be thrown off. The effects would have enormous repercussions and would be difficult to reverse. It wouldn’t be like gently turning a knob, but rather like turning off a switch. And it wouldn’t be easy to turn it back on.
Mass death of coral reefs
When corals turn ghostly white, it’s not always because they’re dead, and their reefs may not be lost forever yet. When the water gets too hot, corals expel algae that live symbiotically within their tissues. If conditions improve, they can survive this bleaching. After a while, reefs eventually recover. However, as the mercury rises, occasional bleaching is turning into regular bleaching. Moderate bleaching is turning into extreme bleaching.
The latest predictions from experts are grim. Even if humanity moves to control global warming, 70 to 90 percent of the corals currently forming reefs could die in the next few decades. If left unchecked, the toll could be 99 percent or more. A reef may look healthy right up until its corals begin to bleach and die. In the end, it’s a graveyard.
This is not to say that it is inevitable that reef-building corals will go extinct. The hardiest may survive in groups. But the vibrant ecosystems these creatures support will be unrecognizable. A quick recovery is not likely, not in the areas where corals live today, and on any scale.
When could it happen? It may already be happening.
Abrupt thawing of permafrost
In the soil beneath the coldest places on earth, the accumulated remains of long-dead plants and animals contain a lot of carbon — about twice as much as is currently in the atmosphere. When heat, wildfires and rain thaw and destabilize frozen soil, microbes get to work, converting this carbon into carbon dioxide and methane. These greenhouse gases exacerbate the heat, fires and rain, which in turn intensify the thawing.
Like many of these vast, self-propelled changes in our climate, permafrost thawing is hard to predict. Huge areas have already thawed in western Canada, Alaska and Siberia. But it’s hard to estimate how quickly the rest will thaw, how much worse global warming will become and how much carbon will be trapped in the soil by new vegetation sprouting on top of it after the thaw.
When could it happen? It will vary from place to place. The effects on global warming could accumulate over a century or more.
Greenland ice disappears
The colossal ice sheets covering Earth’s poles aren’t melting the way an ice cube melts. Because of their tremendous size and geometric complexity, multiple factors determine how quickly the ice melts and contributes to ocean rise. Scientists are particularly concerned about the sheets that might contribute to their own melting and accelerate the rate at which they melt, as this process would be very difficult to stop.
In Greenland, the problem is elevation. As the ice surface becomes lower, a larger proportion of the ice sheet is at a warmer altitude, exposed to warmer air. This causes it to melt faster.
Thanks to the geological evidence available, scientists know that large parts of Greenland were once ice-free. They also know that the consequences of another major melt could be felt around the world, affecting ocean currents and rainfall all the way to the tropics and beyond.
When could it happen? Irreversible melting could begin this century and continue for several hundred or even thousand years.
Breaking up of ice in the western Antarctic region
Many West Antarctic glaciers slide into the sea, which means that their undersides are exposed to the constant flow of ocean currents. As the water warms, these floating ice caps melt and weaken from the bottom, particularly the part that rests on the seabed. Like a ballerina trying to hold a difficult pose, the ice cap begins to lose its balance. With less ice floating around it to hold it in place, more ice from the continent’s interior will end up in the ocean. Eventually, the ice left at the water’s edge could no longer support its own weight and break into pieces.
The West Antarctic Ice Sheet has likely broken up before, in Earth’s distant past. Scientists are still trying to determine how close the ice is to suffering the same fate today.
When could it happen? As in Greenland, the ice sheet could begin to break up irreversibly this century.
Sudden change in the West African monsoon
Scientists now know that the Sahara has repeatedly changed from arid to humid to dry to temperate. There is less certainty about how the West African monsoon might change or intensify in response to current warming, or even if it will do so at all (despite its name, the West African monsoon brings rain to parts of East Africa as well).
Whatever happens, it will be very important for an area of the world where many people’s nutrition and livelihoods depend on the skies.
When could it happen? It’s hard to predict.
Loss of the Amazon rainforest
In addition to being home to hundreds of indigenous groups, tens of thousands of plant and animal species, and 400 billion trees; in addition to containing untold numbers of other living things that we have yet to discover, name, or describe; and in addition to storing an abundant amount of carbon that might otherwise contribute to global warming, the Amazon rainforest plays another very important role: it is a living, active driver of the climate.
The combined exhalations of all those trees create clouds filled with moisture. When all this moisture falls, it helps keep the region lush and dense.
Unfortunately, farmers and ranchers are now cutting down trees, and global warming is making fires and droughts worse. Scientists worry that when much more rainforest is lost, this rain machine will break down, causing the rest of the rainforest to wither and degrade into grassy savannah.
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Recently, some scientists have calculated that by 2050, up to half of the current Amazon rainforest could be at risk of suffering this type of degradation.
When could it happen? It will depend on how quickly people cut down or protect the rest of the forest.
Elimination of Atlantic currents
A huge torrent of seawater flows through the Atlantic Ocean. It begins on the west coast of Africa, flows through the Caribbean, rises towards Europe and then flows back down to start the cycle again. Along the way, it determines the temperature and precipitation of a large part of the globe. The saltiest and densest water sinks to the depths of the ocean, while the freshest and lightest water rises, in a dynamic that keeps this conveyor belt moving.
But now melting ice from Greenland is pouring huge new flows of fresh water into the North Atlantic, upsetting this balance. Scientists fear the engine could slow down and eventually stall, dramatically altering weather patterns for billions of people in Europe and the tropics.
Scientists have already seen signs of a slowdown in these currents, which have a complex name: the Atlantic meridional overturning circulation, or AMOC.
When could it happen? It is very difficult to predict.
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